This is a cryptic definition - the entire clue serves as both definition and wordplay, with words that can be interpreted in multiple ways to lead to the answer.
Think about different meanings of 'pupil' - not just a student. What kind of barrier naturally drops down to protect this other type of pupil?
Answer: EYELID | This is a cryptic definition where 'pupil' refers to the pupil of an eye rather than a school pupil. An eyelid is literally a barrier that drops to protect the pupil of the eye.